1. Field of the Invention
This invention relates to a process for the selective production of para-xylene by catalytic methylation of toluene at a specified elevated temperature.
2. Description of the Prior Art
Alkylation of aromatic hydrocarbons utilizing cystalline aluminosilicate catalysts has heretofore been described. U.S. Pat. No. 2,904,697 to Mattox refers to alkylation of aromatic hydrocarbons with an olefin in the presence of a crystalline metallic aluminosilicate having uniform pore openings of about 6 to 15 Angstrom units. U.S. Pat. No. 3,251,897 to Wise describes alkylation of aromatic hydrocarbons in the presence of X- or Y-type crystalline aluminosilicate zeolites, specifically such type zeolites wherein the cation is rare earth and/or hydrogen. U.S. Pat. No. 3,751,504 to Keown et al. and U.S. Pat. No. 3,751,506 to Burress describe vapor phase alkylation of aromatic hydrocarbons with olefins, e.g. benzene with ethylene, in the presence of a ZSM-5 type zeolite catalyst.
The alkylation of toluene with methanol in the presence of a cation exchanged zeolite Y has been described by Yashima et al. in the Journal of Catalysis 16, 273-280 (1970). These workers reported selective production of para-xylene over the approximate temperature range of 200.degree. to 275.degree.C., with the maximum yield of para-xylene in the mixture of xylenes, i.e. about 50 percent of the xylene product mixture, being observed at 225.degree.C. Higher temperatures were reported to result in an increase in the yield of meta-xylene and a decrease in production of para and ortho-xylenes.
While the above-noted prior art is considered of interest in connection with the subject matter of the present invention, the methylation process described herein carried out at a temperature between about 500.degree.C. and about 750.degree.C. utilizing a catalyst of a crystalline aluminosilicate zeolite having a silica/alumina ratio of at least about 12 and a constraint index within the approximate range of 1 to 12 to achieve unexpectedly high selective production of para-xylene has not, insofar as is known, been heretofore described. Indeed, such finding would appear to be directly contrary to expectations in light of the prior art.
Of the xylene isomers, i.e., ortho-, meta- and para-xylene, the latter is of particular value being useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers such as "Dacron". Mixtures of xylene isomers either alone or in further admixture with ethylbenzene, generally containing a concentration of about 24 weight percent para-xylene in the equilibrium mixture, have been previously separated by expensive superfraction and multistage refrigeration steps. Such process, as will be realized, has involved high operation costs and has a limited yield.